256 MB might be a bit much for the 3DS RAM (although considering the higher requirements for 3-D processing, it's possible that more RAM than usual could be necessary.)
I'd expect something between 128 and 256 MB, considering Nintendo never really uses standardized numbers (Like how the GC and Wii both have weird numbers like 43 MB and 88 MB of RAM, and it's all spread out over the console to the areas that need it).
I wonder if they'll still use that MoSys 1T-SRAM that they've used since the GC. It's pretty good stuff, although more expensive than the usual RAM, mainly due to it's low-power consumption. I'm thinking they will for precisely that reason, but I wouldn't expect something beyond 256 MB at the absolute max, which is sort of overkill for the kinds of stuff that the 3DS will play.